For long-haul optical transmission, the link accumulated optical noise consists of linear and nonlinear contributions. The linear noise results from optical amplification, that is, amplified spontaneous emission (ASE) noise. An important nonlinear noise results from the Kerr effect in optical fiber. The ratio of linear and nonlinear noise depends on the power of optical signals during transmission. At small launch power, the link accumulated noise is dominated by linear noise due to low optical signal-to-noise ratio (OSNR). At high launch power, the fiber nonlinearities can dominate. The highest SNR, which includes other power-independent noise such as transceiver internal noise, is achieved at a launch power where a fixed proportion of linear and nonlinear optical noise is approximately 2 to 1.
Separate measurement of linear and nonlinear optical noise provides a means of optical power optimization, capacity estimation, and capacity maximization. However, it is not straightforward to distinguish linear and nonlinear noise in the time domain. Furthermore, direct OSNR measurement in the frequency domain becomes impractical as the available bandwidth of optical fibers is completely occupied by signals for higher spectral efficiency.
It has been proposed in Zhenhua Dong, Alan Pak Tao Lau, and Chao Lu, “OSNR monitoring for QPSK and 16-QAM systems in presence of fiber nonlinearities for digital coherent receivers,” Opt. Express 20, 19520-19534 (2012) and in H. G. Choi, J. H. Chang, Hoon Kim and Y. C. Chung, “Nonlinearity-tolerant OSNR estimation technique for coherent optical systems,” 2015 Optical Fiber Communications Conference and Exhibition (OFC), Los Angeles, Calif., 2015, Paper W4D.2 to use the correlation of amplitude noise or symbol amplitude on received symbols to derive the nonlinear noise. However, accuracy of results depends on net chromatic dispersion (CD), fiber type, fiber length, inline CD compensation and so forth.